1. Field of the Invention
The present invention relates to a driving method for an electric vehicle and, more particularly, to a power distribution method for an electric vehicle driven by two power sources.
2. Description of the Related Art
Vehicles in the current market used to consume fuels as the main power source. Responsive to the global oil shortage, extensive researches in substitute energy vehicles and hybrid vehicles have been made in industries with an intention to minimize the adverse effect of the oil shortage on the vehicle markets. Every car company actively tried to replace oil with electricity as the power sources for vehicles and developed various electric vehicles and various hybrid vehicles to gradually get rid of reliance of oil, and electricity has been a successful power source and gains a role in the market.
Conforming to this trend the electric vehicle industry develops flourishingly, but thus, many technical problems occurred. The problems should be solved in electric vehicles driven by motors significantly different from conventional vehicles driven by internal combustion engines. For electric vehicles, a transmission is required for the electric motor to output an extremely high torque while the vehicle moves at a low speed and to obtain an extremely high rotating speed while the vehicle moves at a high speed. Nevertheless, the transmission must have high efficiency and low costs to provide the product with high competitiveness on the market.
Conventional transmissions for electric vehicles include continuously variable transmissions (CVT), manual transmissions (MT), and automated manual transmissions (AMT). Continuously variable transmissions have low transmission efficiency due to slips during gear shifting processes. Manual transmissions having the highest transmission efficiency but tire the drivers and are inconvenient in use. Thus, automated manual transmissions are a better option. However, automated manual transmissions are expensive and, when a limited number of gears are provided, may result in uncomfortable feel to the drivers due to torque holes during gear shifting processes.
With reference to FIG. 1, to solve the above disadvantages, Taiwan Patent Publication No. 201242804 entitled “Device of twin source power for electric vehicle” discloses an electric vehicle 1 driven by two power sources. The electric vehicle 1 includes a first motor 11, a second motor 12, and a transmission 13. The transmission 13 is a clutchless automatic manual transmission (CLAMT). The power of the first motor 11 is directly transmitted to a transmission shaft 14. The power of the second motor 12 is transmitted to the transmission shaft 14 after gear shifting by the transmission 13. The power received by the transmission shaft 14 is a sum of the powers outputted by the first and second motors 11 and 12. The total power demanded by the electric vehicle 1 is shared by the first and second motors 11 and 12. The transmission 13 includes an automatic gear shifting mechanism. Electronic synchronization is firstly executed when a vehicle control unit 15 (VCU) gives a gear shifting command, and a motor driver 121 of the second motor 12 controls the rotating speed of the second motor 12 such that the driving gear and the driven gear of the transmission 13 rotate synchronously. A gear shifting driver 131 drives a gear shifting actuator 132 to actuate the automatic gear shifting mechanism for automatically changing the gear.
Since the electric vehicle 1 includes an electronic synchronization function, the transmission 13 does not require a conventional clutch device, significantly saving the costs and the space for installation. Furthermore, the first motor 11 shares a portion of the power and, thus, can increase the output power when the second motor 12 does not output power during the gear shifting process. The torque hole of the second motor 12 is compensated to provide a smooth vehicle power output during the gear shifting process, increasing driving comfort.
According to the regulations to electric vehicles in many countries, the total power of the power sources of micro electric vehicles and low speed electric vehicles must be lower than 4 KW. Nevertheless, multi-seat electric vehicles meeting the regulations must be able to travel on various grades under a heavy load and able to travel on a level road at a maximum speed of at least 45 km/hr. Thus, under the limitation of total power of the electric vehicle 1, distribution of the power outputs of the first and second motors 11 and 12 of the electric vehicle 1 for operating the first and second motors 11 and 12 at the most efficient operation points and making the transmission 13 select a proper gear according to actual load conditions for satisfying driver's demand and reducing energy consumption are difficult issues to be solved.
Thus, a need exists for a power distribution method for an electric vehicle driven by two power sources to properly arrange the power output of each power source of a conventional electric vehicle driven by two power sources such that each power source can be operated at the most efficient operation point to effectively increase the operational efficiency of the conventional electric vehicle driven by two power sources and, at the same time, to control the gear of the transmission according to the vehicle load so as to provide a high torque output at low speeds while meeting the extreme speed requirement under limited power of the power sources, increasing the climbing performance, load capacity, operational convenience, and driving comfort of the electric vehicle having a limited total power.